1. Field of the Invention
The invention relates to a mobile radio set comprising a speech processing device for processing speech signals formed by noise components and speech components.
2. Description of the Related Art
In the field of speech processing, speech signals to be processed often contain noise components, which leads to a degradation of the speech quality and thus especially to degraded intelligibility. This problem occurs especially in mobile radio sets which are used in private cars that have a hands-free facility. Speech signals received from microphones of the hands-free facility in the private car contain, on the one hand, speech components generated by the user (speech source) of the mobile radio set inside the private car and, on the other, noise components which are the result of further ambient noise of the mobile radio set. During a ride the ambient noise consists, in essence, of engine and driving noises.
From "Proceedings of the IEEE, Vol. 75, No. 2, February 1987" is known a device comprising a plurality of microphones in which, except for one microphone signal, all further microphone signals are applied to adjustable delay elements. The microphone signal shifted with time relative to each other by the delay elements, are added together and subjected to further processing. The useful signal components of the microphone signals originate, in essence, from a single acoustic source which is at different distances from the microphones.
Thus, for an acoustic signal generated by the acoustic source there are different delays to the spatially arranged microphones. The acoustic signal is the source of time-shifted but otherwise substantially identical useful signal components of the microphone signals. The useful signal components are thus strongly correlated. Noise components of the microphone signals, however, are at most slightly correlated when the microphones are suitably arranged. A suitable setting of the delay elements with respect to the position of the acoustic source thereby improves the output signal of the device as to its signal-to-noise ratio.
Such a device provides satisfactory results only if the signal-to-noise ratio of the microphone signals to be processed lies above a threshold; i.e. the useful signal components are large enough compared with the noise components. Especially; the noise components must not be greater than the useful signal components. For this reason an estimate must be made of the signal-to-noise ratio of at least one microphone signal each time the delay elements are reset, so that erroneous functioning of the speech processing device is avoided when the signal-to-noise ratio is insufficient.
Devices used to date for determining the signal-to-noise ratio of a speech signal formed by noise components and speech components determine a value for the noise power during a speech pause when only noise components occur. The detection of a speech pause is based, for example, upon a statistical evaluation of the speech signal by histograms or upon evaluation of the short-term power of the noisy speech signal.
Such a speech-pause dependent determination of the signal-to-noise ratio is, on the one hand, susceptible to error because the speech pause needs to be detected and, on the other hand, slow because the signal-to-noise ratio can only be updated when there is a speech pause, whereas in between the speech pauses the power of the noise component may have changed.